The present invention relates to diffraction optics screen apparatus, and more particularly to such apparatus which provides the capabilities of full-color viewing and suppression of zero-order light.
Diffraction optics diffusion screens employing holographic elements are well known in the art. U.S. Pat. No. 4,372,639 discloses a diffraction optics directional diffusing screen. FIG. 1 illustrates a simple diffraction optics diffusion screen 20 of the type more particularly described in U.S. Pat. No. 4,372,639, illuminated by light from a monochromatic light source 22. This type of screen employs a diffusion hologram, and exhibits a small and well-defined exit pupil, high and uniform gain across the exit pupil, and low backscatter. However, the light which passes through the holographic optical element 20 without diffraction, known as zero-order light, passes into the ambient, which is undesirable. Further, the simple diffusion hologram of FIG. 1 does not play back well in full-color. The white light incident on the hologram would be dispersed, i.e., diffracted at different angles for different wavelengths, creating a rainbow-like fan of colored light exiting each point on the hologram. This effect is illustrated in FIG. 2, wherein the screen 20 is illuminated by light from a white light source 24. Because the divergence of the projected beam onto the screen creates a wide variation in the incident angle, the resultant dispersion is so great that only a small overlap region between the red, green, and blue exit pupils is available for full-color viewing.
The problem of the zero-order light can be addressed by the addition of a fiber optic faceplate, as shown in U.S. Pat. No. 4,586,781. The faceplate absorbs the zero-order light while transmitting the diffracted light, and scrambles the multicolored diffuse cones of light about its fiber axis to create a uniform full-color exit cone. The problem with this configuration is that the optical fibers of a fiber optic faceplate are all oriented parallel to each other, so that the exiting diffuse cones are all parallel to one another. To obtain any appreciable overlapped full-color viewing region requires making the diffuse cones very large. The result is that much of the light is wasted and the main advantage of the holographic screen, its high gain, is severely compromised. One could use a lens on the front of the screen to achieve the focusing of the diffuse cones so as to preserve the high screen gain. However, a convex surface facing outward in a high ambient level environment is unacceptable, because it will reflect ambient light or sunlight from a wide range of positions directly into the viewer's exit pupil.
It is therefore an object of this invention to provide a screen possessing the advantages of the directional diffraction optics diffusing screen, and in addition be able to block the zero-order beam and allow full-color viewing.